CODAR’s unique, proprietary call-sign algorithm — CallSign™ — is designed to fulfill the International
Telecommunications Union (ITU) requirement that each oceanographic HF radar routinely issue a unique
Morse code identifier signal within it transmissions.
Traditionally, call signs for radio broadcasts are sent out during periods when normal transmission is halted. This practice of
suspending normal transmissions for issuing a call sign is not an option for HF radar users who rely on a continuous data
stream for supporting emergency response activities like search and rescue, oil spill tracking, or tsunami detection, as the
result of would be recurring instances of large data gaps. CODAR’s patent-pending call-sign routine sends the Morse code
signal ingeniously without adversely affecting normal SeaSonde ocean monitoring processes.
CallSign allows each radar to maintain continuous Doppler signal processing. When a region in a time series is zeroed out to
make way for a call-sign transmission, the impulsive nature of on/off blanking around the gap causes digital sideline noise
throughout the Doppler spectrum. To mitigate this digital interference, an interpolation scheme in CallSign replaces the gap
signal for each range cell with a time-series average that eliminates the gap. With this solution, the call sign can be placed
anywhere in the time series, and, unlike the less elegant approach of stalling computations, signal processing remains
continuous.
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| Time series with indigo vertical band blanked for call sign transmission, range (y-axis) vs. time (x-axis) vs. intensity (color bar) (above left).
Resulting Doppler spectrum with sideband noise manifesting as slanted streaks, range (y-axis) vs. frequency (x-axis) vs. intensity (color
bar) (above right). |
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| Time series augmented with averaged data (above left), and resulting, uncorrupted Doppler spectrum (above right). |
Because of ITU requirements, numerous SeaSondes operating in proximity to
each other as part of a network typically use the same frequency. CODAR has
tested its call-sign broadcast to ensure that the call sign’s presence is not seen
by these nearby SeaSonde radars in the network, meaning it falls below their
noise floors and does not impact their performance. However, the callsign
signal intensity and bandwidth are designed to match SeaSonde intensity and
bandwidth so that any adversely impacted radio receivers will hear the
identifier signal.
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| Graphical User Interface for CallSign setup |
CallSign’s operating parameters are easily set inside its graphical user
interface. Setting up call-sign transmission function is as simple as entering its
designation and desired scheduling parameters, typically set to transmit every
20 minutes and not precisely on the hour. User can also select whether signal
is to be sent more than one time in sequence thereby providing listeners with
multiple opportunities to capture it. Different call signs for neighboring sites
that share frequency band can be scheduled one or two minutes apart from
each other to avoid issuing overlapping, undecipherable call-sign signals.
Look for CODAR’s CallSign in upcoming Radial Suite Release 8.
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